1. Field of the Invention
The present invention relates to a gas or liquid seal and, more specifically, to a compliant seal for sealing high pressure areas from low pressure areas.
2. Description of the Related Art
Many different types of machinery use clearance seals to contain a gas or other fluid under pressure. However, varying conditions during machine operation present many difficulties when designing these clearance seals. For example, when a machinery shaft rotates it may undergo a slight centrifugal growth at high rotational speeds. In addition, during rotation these shafts may undergo radial excursions caused by extreme speed changes, temperature fluctuations, non-concentric alignment, shaft runout, or other factors. Thus, a clearance seal between a rotating machine shaft and casing must adapt to these dynamic conditions.
Typical clearance seals include simple labyrinth seals which break down pressure by using a series of teeth or blades in close proximity to a rotating shaft. Such devices are well known and reliable, but allow high rates of gas leakage. Typically, clearances between the blade tips of a labyrinth seal and the shaft are as high as 0.001-0.002 inches per inch of shaft diameter.
Much work has been done to reduce the leakage of labyrinth seals. Typically, the approach of past efforts has been directed toward improved blade geometry (e.g., backward angled blade designs), step seals, or interlocking blade designs to reduce losses. Abradable labyrinth seal designs have also been developed and are employed, particularly, on jet engines. The blades of such abradable seals are initially installed with a line-to-line clearance, and then wear in to their own clearance as the machine rotates. However, after the rotor has gone through an operating history of radial vibration combined with axial excursions, a crescent-shaped clearance is often cut into the abradable seal, and leakage can increase dramatically. In spite of many improved designs, therefore, leakage losses for labyrinth seals continue to represent a considerable energy loss in rotating machinery.
Another significant problem with high pressure labyrinth seals is the occurrence of rotordynamic effects. Since high pressure gas is quite dense, a labyrinth seal begins to act as a bearing with high gas pressures, but tends to destabilize the rotor due to a lack of concentricity with the actual machine bearings. Significant effort has been made recently to decrease rotordynamic excitation effects. The product of these efforts includes swirl breaks in front of a labyrinth, reverse swirl inducers, and injecting tangentially non-moving gas into the first stagnant region of the labyrinth. Other work has been concentrated on optimized geometry and creating a rough surface within the labyrinth seal to retard circumferential gas velocity buildup. Also, honeycomb design labyrinth seals have been developed to reduce rotordynamic excitation effects. However, further reduction of rotordynamic effects is needed.
As an alternative to the labyrinth seal, applicant has proposed a seal having flexible elements which ride upon a thin gas or liquid film between the fingers and a rotating shaft. Such compliant seals are disclosed in U.S. Pat. No. 5,100,158 and U.S. Pat. No. 5,370,402, the disclosures of which are incorporated by reference.